A current mirror is a basic building block of circuitry, particularly in current-mode circuits. A current mirror receives a current and generates a matching current. A current mirror has a wide variety of applications including digital to analog converter (DAC), automatic gain control, tunable time filter, etc.
FIG. 1 shows a basic current mirror. It comprises field effect transistor (FET) 102 which has its drain and gate coupled together and FET 104 which has its gate coupled to the gate of FET 102. When current flows through FET 102 the voltage registered at the gate of FET 102 controls the current through FET 104 to a matching current to that flowing through FET 102. As a result, the input current IREF. is mirrored at output node 106 by output current IMIRROR. Current mirrors are can also be constructed from bipolar junction transistors (BJTs) in a similar fashion.